A magnetic data-recording/reproducing apparatus has a mechanism for driving reel mounts at high speed so as to transfer magnetic tape from one reel to another. This mechanism has the structure shown in FIG. 1. As is shown in the figure, S reel (supply) and T reel (takeup) (neither shown) are removably placed on a tape supply (S) reel mount 1 and a tape take-up (T) reel mount 2, respectively. To transfer tape fast, one end of the magnetic tape 3 is drawn from the S reel, guided through a predetermined path, and wrapped around a cylinder 4 which has a magnetic head mounted on it, and taken up around the T reel. In other words, the tape 3 is transferred in the method known as full-loading, fast transfer. Depending on whether the magnetic tape 3 is to be transferred forward or rewound at high speed, either the S reel mount 1 or the T reel mount 2 is rotated by an idler 7 which in turn is rotated by a motor pulley 6 connected to a reel motor 5, which in most cases is driven by a constant voltage. Fast transfer or rewinding of the tape can, however, be accomplished by means other than the reel motor 5; in some cases, it can be achieved by transmitting the drive force from a capstan motor (not shown) or a cylinder motor (not shown) to the S reel mount 1 or the T reel mount 2.
A soft brake 8 is provided beside the driven reel mount 1, and applies a tension greater than a predetermined value to the magnetic tape 3 when the tape is being transferred at high speed, so as to prevent the tape 3 from slackening. The soft brake 8 comprises a support pin 9, a brake lever 10 rotatably supported by the pin 9, a brake shoe 11 located at one end of the brake lever 10 and normally in contact with the outer circumferential surface of the driven reel mount (in the figure, the S reel mount 1), and a spring 12 attached to the other end of the brake lever 10 and urging the brake shoe 11 against the outer circumferential surface of the reel mount.
In the conventional reel-mount driving mechanism described above, the brake torque of the soft brake 8 is constant, since it is generated by the brake shoe 11 urged by the spring 12 against the outer circumferential surface of the S reel mount 1. FIGS. 2A, 2B, and 2C illustrate how the drive voltage of the reel motor 5, the drive current of the reel motor 5, and the amount of tension applied to the tape change in accordance with the diameter of the tape roll on the reel mount driven by the conventional reel-mount driving mechanism. The minimum tape tension value, shown in FIG. 2C, is determined on the basis of the diameter of the tape roll at the start of the fast transfer or rewinding. The above figure illustrates the tape-tension characteristic which is detected when the tape roll on the driving reel is at the minimum diameter, while the tape roll on the driven reel has the maximum diameter. As the magnetic tape 3 is being transferred from one reel to the other, the diameter of the tape roll on the driving reel increases, and that of the tape roll on the driven reel decreases. Since, as mentioned earlier, the brake torque applied to the driven reel mount is constant, the tension on that portion of the magnetic tape 3 which is being drawn from the driven reel to the driving wheel is a function of the ratio of the brake torque on the driven reel mount to the radius of the tape roll on the driven reel. Therefore, since the radius of the tape roll on the driven reel decreases as the tape 3 is taken up by the driving reel, the tension on that portion of the tape 3 which is about to be taken up around the driving reel gradually increases. The amount of tension further increases due to the tape-guiding path. The torque required for the driving reel to take up the tape, is determined by the ratio of the tension on the tape about to be taken up, to the radius of the tape roll already thereon up. Hence, even if the tension on the tape, which is about to be taken up were to remain unchanged throughout the winding process, the torque required for the tape to be taken up does increase as more and more tape is taken up by the driving reel, since this results in a steady increase in the radius of the tape roll thereon. Due to the increase in the tension applied on the tape being taken up and the resultant increase in the torque required to take it up, the torque required near the end of the tape take-up is therefore far greater than that required at the beginning. As a result, the amount of current which must be supplied to the reel motor 5 driving the driving reel mount increases drastically near the end of tape take-up. Thus, the inner a turn of the tape wound around the reel, the lower the tension applied, whereas the outer the turn, the higher the tension applied.
As has been described, when the conventional reel-mount driving mechanism is used, the innermost turns of the tape wound around a reel are likely to slacken, whereas the outermost turns are likely to be too tense. The minimum tension required to prevent the wound tape from becoming too tense is that tension applied on the tape while the tape is being transferred forward at high speed, or that tension applied on the tape at the beginning of the tape rewinding. Hence, the tension on the tape is higher than necessary at near the end of the tape rewinding, the load on the reel motor 5 increases. Here arise the problem that the motor consumes much power, and its lifetime may decrease. This problem must be solved, particularly when the magnetic tape 3 is transferred forward or rewound at high speed, with its one portion wrapped around the cylinder 4 in a so-called "full-loading" condition, with the result that a considerable load is imposed on the tape in the tape-guiding path.